漏洞简介: 

极光无限维阵专家团队在对Python最新版Python 3.8.5的分析中发现了一个空指针引用漏洞，漏洞发生在Python对.pyc文件进行处理时。

下面是崩溃信息：

$ Python-3.8.5/python 00-SEGV-on-unknown-address-Python-3.8.5.pyc    Could not find platform dependent libraries <exec_prefix>    Consider setting $PYTHONHOME to <prefix>[:<exec_prefix>]    AddressSanitizer:DEADLYSIGNAL    =================================================================    ==8079==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000018 (pc 0x0000008aa86b bp 0x000000000000 sp 0x7ffe2a3f5bf0 T0)    ==8079==The signal is caused by a READ memory access.    ==8079==Hint: address points to the zero page.        #0 0x8aa86a in _PyEval_EvalCodeWithName /home/test/Python-3.8.5/Python/ceval.c:4266:23        #1 0x866d0f in PyEval_EvalCodeEx /home/test/Python-3.8.5/Python/ceval.c:4327:12        #2 0x866d0f in PyEval_EvalCode /home/test/Python-3.8.5/Python/ceval.c:718:12        #3 0x9f7355 in run_eval_code_obj /home/test/Python-3.8.5/Python/pythonrun.c:1125:9        #4 0x9e682d in run_pyc_file /home/test/Python-3.8.5/Python/pythonrun.c:1184:9        #5 0x9e682d in PyRun_SimpleFileExFlags /home/test/Python-3.8.5/Python/pythonrun.c:419:13        #6 0x9e4ca5 in PyRun_AnyFileExFlags /home/test/Python-3.8.5/Python/pythonrun.c:86:16        #7 0x5108db in pymain_run_file /home/test/Python-3.8.5/Modules/main.c:381:15        #8 0x5108db in pymain_run_python /home/test/Python-3.8.5/Modules/main.c:606:21        #9 0x5108db in Py_RunMain /home/test/Python-3.8.5/Modules/main.c:685:5        #10 0x5129d6 in pymain_main /home/test/Python-3.8.5/Modules/main.c:715:12        #11 0x512dd7 in Py_BytesMain /home/test/Python-3.8.5/Modules/main.c:739:12        #12 0x7f8316d4b82f in __libc_start_main /build/glibc-LK5gWL/glibc-2.23/csu/../csu/libc-start.c:291        #13 0x438888 in _start (/home/test/Python-3.8.5-Fuzz/python+0x438888)    AddressSanitizer can not provide additional info.    SUMMARY: AddressSanitizer: SEGV /home/test/Python-3.8.5/Python/ceval.c:4266:23 in _PyEval_EvalCodeWithName    ==8079==ABORTING

漏洞分析: 

在函数PyEval_EvalCode(Python-3.8.5/Python/ceval.c)中调用PyEval_EvalCodeEx函数。

 PyObject *    PyEval_EvalCode(PyObject *co, PyObject *globals, PyObject *locals){        return PyEval_EvalCodeEx(co,                        globals, locals,                        (PyObject **)NULL, 0,                        (PyObject **)NULL, 0,                        (PyObject **)NULL, 0,                        NULL, NULL);    }

传递给PyEval_EvalCodeEx函数的参数中closure设置为NULL。

    PyObject *    PyEval_EvalCodeEx(PyObject *_co, PyObject *globals, PyObject *locals,                    PyObject *const *args, int argcount,                    PyObject *const *kws, int kwcount,                    PyObject *const *defs, int defcount,                    PyObject *kwdefs, PyObject *closure){        return _PyEval_EvalCodeWithName(_co, globals, locals,                                        args, argcount,                                        kws, kws != NULL ? kws + 1 : NULL,                                        kwcount, 2,                                        defs, defcount,                                        kwdefs, closure,                                        NULL, NULL);    }

PyEval_EvalCodeEx函数继续调用_PyEval_EvalCodeWithName函数，closure值不变依旧为NULL。

    PyObject *    _PyEval_EvalCodeWithName(PyObject *_co, PyObject *globals, PyObject *locals,            PyObject *const *args, Py_ssize_t argcount,            PyObject *const *kwnames, PyObject *const *kwargs,            Py_ssize_t kwcount, int kwstep,            PyObject *const *defs, Py_ssize_t defcount,            PyObject *kwdefs, PyObject *closure,            PyObject *name, PyObject *qualname)    {        ******        /* Copy closure variables to free variables */        for (i = 0; i < PyTuple_GET_SIZE(co->co_freevars); ++i) {            PyObject *o = PyTuple_GET_ITEM(closure, i);   <----------------------------- crash            Py_INCREF(o);            freevars[PyTuple_GET_SIZE(co->co_cellvars) + i] = o;        }        ******    }

修复建议: 

在 _PyEval_EvalCodeWithName函数中引用closure前，对closure的值进行判断。